The present invention relates to: a water base adhesion promotor for improving the paint adhesion to polypropylene materials; and a method for coating to polypropylene materials using the promoter.
Polyolefins such as polypropylene are comparatively cheap and has excellent properties such as formability and chemical resistance, so they are widely used in the industrial fields including the automobile industry. The amount thereof as used tends to increase, because their recycling is possible and because their prices are low. Polypropylene materials are, for example, used for exterior decoration parts of automobiles, such as bumpers and molds, and their surfaces are usually coated to both improve their weather resistance and give the design proper thereto.
The polypropylene material is inferior in wettability. Therefore, when being coated with a paint the polypropylene material undergoes the following treatments: first, a degreasing treatment with a water base cleaning agent and a pretreatment to enhance the wettability using a surface-active agent, and then coating and baking treatments of a primer, and then a coating treatment of a paint. Incidentally, the coating treatment of the primer is carried out to enhance the adhesion of paints, but, in fact, as much as about 70% of the amount of the primer as used is lost during application and therefore is not effectively used, thus resulting in high coating cost. Furthermore, there are problems in that: the primer needs to be baked after being coated, so a large baking furnace is needed and the equipment cost is therefore high.
From the above circumstances, it is desired to develop a method for coating to polypropylene materials without primer coating.
An object of the present invention is to provide: a water base adhesion promotor to enhance the paint adhesion to polypropylene materials instead of the primer coating; and a method of coating to polypropylene materials using the promotor.
The present inventors made various examinations and experiments to solve the above problems. As a result, they attained the present invention by finding that: a composition, comprising a grafted polypropylene chloride of high adhesion to polypropylene materials, an amine-neutralized water-soluble resin of high water resistance, and a wettability-improving agent, could enhance the paint adhesion to polypropylene materials, so that the primer coating could be omitted.
That is to say, a water base adhesion promotor for polypropylene, according to the present invention, comprises:
a grafted polypropylene chloride, which includes a polypropylene chloride moiety and a maleic anhydride moiety as bonded thereto, and which has a chlorine content of 15xcx9c25 weight % and a maleic anhydride moiety content of 1xcx9c5 weight %;
an amine-neutralized water-soluble resin;
a wettability-improving agent; and
water;
and is characterized by having:
a combination ratio of 25xcx9c90 weight % as of the grafted polypropylene chloride to the total of the grafted polypropylene chloride and the amine-neutralized water-soluble resin;
a combination ratio of 2.5xcx9c6.0 weight % as of the wettability-improving agent to the entirety of the promotor; and
a solid content of 2xcx9c10 weight % of the entirety of the promotor.
A method for coating to polypropylene materials, according to the present invention, comprises:
a pretreatment step in which a polypropylene material is beforehand cleaned, and then the above water base adhesion promotor is applied, then dried; and
a coating step in which a paint is coated to the material after the pretreatment step.
Hereinafter, each component, constituting the present invention water base adhesion promotor for polypropylene, is explained in detail, and then the water base adhesion promotor for polypropylene and the method for coating to polypropylene materials are explained.
(Grafted Polypropylene Chloride)
The grafted polypropylene chloride, as used in the present invention, is a component to enhance the adhesion to polypropylene materials. The grafted polypropylene chloride is a polypropylene derivative which includes a polypropylene chloride moiety and a maleic anhydride moiety as bonded thereto, and is usually used in the form of an emulsion having the high handling property.
The polypropylene chloride moiety is a moiety comprising a polypropylene having at least one chlorine atom as a substituent. In addition, the maleic anhydride moiety is a moiety as obtained by grafting maleic anhydride, and has a cyclic acid anhydride group (deriving from maleic anhydride) and/or two free carboxyl groups.
The grafted polypropylene chloride is a product by reacting a polypropylene upon maleic anhydride and chlorine to internally graft the polypropylene, and is, for example, produced by reacting chlorine and maleic anhydride upon a polypropylene. Incidentally, whichever of chlorine and maleic anhydride may be reacted earlier than the other. The reaction with chlorine is, for example, carried out by introducing a chlorine gas into a solution including a polypropylene. In addition, the reaction with maleic anhydride is, for example, carried out by reacting maleic anhydride upon a polypropylene (or its chloride) in the presence of a peroxide.
The chlorine content in the grafted polypropylene chloride is in the range of 15xcx9c25 weight %, preferably 18xcx9c12 weight %. In the case where the chlorine content is less than 15 weight %, the melting point of the grafted polypropylene chloride is so high that the crystallization easily occurs and that the emulsification is difficult, thus resulting in depression of the storage stability of the resultant water base adhesion promotor. On the other hand, in the case where the chlorine content exceeds 25 weight %, the grafted polypropylene chloride is noncrystalline, thus resulting in depression of the adhesion and the water resistance.
The maleic anhydride moiety content in the grafted polypropylene chloride is in the range of 1xcx9c5 weight %, preferably 3xcx9c45 weight %. In the case where the maleic anhydride moiety content is less than 1 weight %, the emulsification is difficult, thus resulting in depression of the storage stability of the resultant water base adhesion promotor. On the other hand, in the case where the maleic anhydride moiety content exceeds 5 weight %, the acid anhydride group content is too much, thus resulting in depression of the adhesion and the water resistance.
The weight-average molecular weight of the grafted polypropylene chloride is not especially limited, but is preferably in the range of 20,000xcx9c100,000, more preferably 30,000xcx9c80,000. In the case where the weight-average molecular weight is less than 20,000, the promotor might swell with a solvent for a top paint, so that the adhesion might be deteriorated. On the other hand; the weight-average molecular weight exceeds 100,000, the viscosity of the resultant water base adhesion promotor might be so high that the emulsification might be difficult and that the storage stability might be low.
(Amine-neutralized Water-soluble Resin)
The amine-neutralized water-soluble resin, as used in the present invention, is a resin as water-solubilized by neutralization with an amine, and is a film-formable component having high water resistance. Incidentally, for example, water-soluble resins, which are not obtained by the amine neutralization, such as polyethylene glycol and polyvinylpyrrolidone, merely have low water resistance, because, even after film formation by drying, such water-soluble resins remain and retain its water solubility. In comparison, the amine-neutralized water-soluble resin, as used in the present invention, displays high water resistance, because it volatilizes the amine in the drying step, and thus loses the water solubility.
The amine-neutralized water-soluble resin, as used in the present invention, is not especially limited if it is a resin as neutralized with an amine, and examples thereof include products by neutralization of resins such as urethane resins, alkyd resins, and acrylic resins. These may be used either alone respectively or in combinations with each other.
Examples of the amine, as used for the above neutralization, include ammonia, triethylamine, and monoethanolamine. These may be used either alone respectively or in combinations with each other.
(Wettability-improving Agent)
The wettability-improving agent, as used in the present invention, is a component to enhance the wettability of the water base adhesion promotor upon polypropylene materials (having much water-repellency). The use of this wettability-improving agent enables a coating film, resultant from the grafted polypropylene chloride and the amine-neutralized water-soluble resin as above, to adhere well to polypropylene materials.
The wettability-improving agent is not especially limited, but examples thereof include ethylene glycol hexyl ether, dimethylpolysiloxane-polyethylene glycol block polymer (Polyflow KL-245). These may be used either alone respectively or in combinations with each other. Particularly, ethylene glycol hexyl ether improves the wettability, and further does not lower the water resistance because it volatilizes in the drying step and therefore does not remain in the resultant coating film. In comparison, materials as conventionally used to enhance the wettability, such as alkylphenylpolyethylene glycol, do not volatilize in the drying step, but remain in the coating film, thus depressing the water resistance.
(Other Components)
The water base adhesion promotor of the present invention may further comprise the following other components: resin emulsions such as acrylic emulsions, urethane emulsions, and polyvinyl chloride emulsions; inorganic pigments such as titanium white and carbon black, extenders such as talc and precipitated barium; and various organic color pigments.
(Water Base Adhesion Promotor for Polypropylene and Method for Coating)
The water base adhesion promotor for polypropylene, according to the present invention, needs to comprise the grafted polypropylene chloride, the amine-neutralized water-soluble resin, the wettability-improving agent, and water (these components are explained in detail above), and may further comprise the above other components if necessary. Conditions, such as order, for mixing these components are not especially limited.
The combination ratio of the grafted polypropylene chloride, as contained in the present invention water base adhesion promotor for polypropylene, is in the range of 25xcx9c90 weight %, preferably 30xcx9c50 weight %, of the total (the total solid content) of the grafted polypropylene chloride and the amine-neutralized water-soluble resin. In the case where the combination ratio of the grafted polypropylene chloride is less than 25 weight %, the adhesion and the water resistance are both low. On the other hand, in the case where the combination ratio of the grafted polypropylene chloride exceeds 90 weight %, the adhesion and the water resistance as well as the film formability are low.
The combination ratio of the wettability-improving agent is in the range of 2.5xcx9c6.0 weight %, preferably 3.0xcx9c4.0 weight %, of the entirety of the water base adhesion promotor. In the case where the combination ratio of the wettability-improving agent is less than 2.5 weight %, the polypropylene material crawls with the water base adhesion promotor, so uniform coating is impossible. On the other hand, in the case where the combination ratio of the wettability-improving agent exceeds 6.0 weight %, the storage stability of the water base adhesion promotor is low, and the grafted polypropylene chloride aggregates to precipitate.
The solid content of the present invention water base adhesion promotor for polypropylene is the nonvolatile matter and is in the range of 2xcx9c10 weight %, preferably 4xcx9c6 weight %, of the entirety of the water base adhesion promoter. In the case where the solid content is less than 2 weight %, the film thickness of the coating film resultant from the water base adhesion promoter is too thin, so the adhesion and the water resistance are both low. On the other hand, in the case where the solid content exceeds 10 weight %, marks of sagging are left after coating the water base adhesion promotor, so the resultant coating film has unevenness, and the appearance is therefore unfavorable.
The method of coating to polypropylene materials, according to the present invention, comprises the pretreatment step and the top coating step.
The pretreatment step includes the steps of cleaning the polypropylene material and applying the above water base adhesion promotor and then drying the material.
The cleaned polypropylene material is, for example, prepared by the following steps (1)xcx9c(3) in the pretreatment step.
(1) A step of spray-washing a polypropylene material with warm water of 60xcx9c80xc2x0 C.
(2) A step of degreasing the polypropylene material with an alkaline cleaning agent after the above step (1).
(3) A step of washing the alkaline cleaning agent off the polypropylene material by spray-washing the polypropylene material with deionized water after the above step (2).
The coating film of the water base adhesion promotor is formed on the surface of the polypropylene material by applying the water base adhesion promotor onto the cleaned polypropylene material and then drying the material. This applying step may be accomplished by, for example, pouring, atomized spraying, dipping and/or showering. The choice of a given applying step for any specific part may be based on economic, process, and quality considerations. For some parts it is preferable to hold the polypropylene material upright after applying step and to then dry the material. For other parts, notably those in which the coating material may pool and/or sag after application, it is preferable to hold the polypropylene material at a slight angle, preferably about 10xcx9c20xc2x0 and most preferably about 15xc2x0 from the vertical during drying.
During the holding step, it is preferred to maintain an atomosphere within the environment to promote even coating after drying, For example, the holding environment may be preferably maintained at a temperature of between about 15xcx9c30xc2x0 C. and about 40xcx9c70% relative humidity and most preferably between 20xcx9c25xc2x0 C. and about 40xcx9c70% relative humidity. This holding step is commonly referred to as flash condition.
The method for drying after the flash condition is not especially limited, and the drying is usually carried out with ovens such as hot-air drying ovens, long-wave infrared ovens, and short-wave infrared ovens at about 60xcx9c120xc2x0 C. for about 5xcx9c15 minutes. The thickness of the dried film, as obtained from the water base adhesion promotor in the above way, is preferably in the range of 0.1xcx9c1 xcexcm.
The coating step includes the step of coating a top paint to the above material after the pretreatment step and is carried out by coating the top paint and then drying it.
The type of the top paint is not especially limited, but examples thereof include one-component melamine baking paints, two-component urethane paints, and one-component lacquer paints.
The method of coating the top paint is not especially limited, but the spray coating is preferable. The method for drying the coated top paint is not especially limited, and the drying is usually carried out with ovens such as hot-air drying ovens, long-wave infrared ovens, and short-wave infrared ovens at 70xcx9c120xc2x0 C. for 20xcx9c60 minutes. The thickness of the dried film, as obtained from the top paint in the above way, is not especially limited, but is preferably in the range of 15xcx9c60 xcexcm.
In the method for coating to polypropylene materials, the use of the above water base adhesion promotor in the pretreatment step enables the omission of the primer coating. Therefore, the productivity in the coating step is largely improved.
Hereinafter, the present invention is more specifically illustrated by the following examples of some preferred embodiments in comparison with comparative examples not according to the invention. However, the invention is not limited to the below-mentioned examples. Hereinafter, the unit xe2x80x9cpart(s)xe2x80x9d is xe2x80x9cweight part(s).xe2x80x9d